Soil and asphalt are compacted by applying energy to the loose material to consolidate it and remove voids, thereby increasing the density of the material and its load bearing capacity. Compaction can be effected with static force such as is exerted by the weight of a nonvibratory road roller, or by impact force such as is exerted by a tamper, or by vibratory force. In compaction machines ordinarily employed in construction work, vibratory force is usually generated by a centrifugal exciter that comprises a rotating eccentric weight. In a vibratory roller, for which the apparatus of the present invention is well suited, a centrifugal exciter is commonly supported in the interior of a roller drum to produce a vibratory action as the drum rolls over the material to be compacted.
The compaction effectivness of a vibratory roller operating upon a given type of material depends upon both the frequency and the force magnitude of the vibration that its exciter generates.
Compaction of soil is most efficiently accomplished with vibration that has both a high frequency and a high force magniture. However, after a few passes it is often desirable to employ a different combination of frequency and force than for the initial soil compaction. For compacting freshly laid asphalt aggregate, the frequency of vibration should be as high as possible, because the higher the frequency the smaller the impact ripple; but too much vibratory force displaces the material, and therefore asphalt aggregate should be initially compacted with only enough vibratory force to consolidate the mixture, the exact amount of force being dependent upon the mixture and the condition of the aggregate. With asphalt compaction, as with soil compaction, the level of force that the exciter generates should be changed after a few passes for optimum results.
Thus a centrifugal exciter for a vibratory roller should be adjustable to provide different frequencies of vibration and magnitudes of centrifugal force, so that the machine will have maximum versatility, to be capable of compacting different materials and of being operated with optimum efficiency at each stage of the compaction of any given material.
The relationship between the parameters involved in the operation of a centrifugal exciter is given by: EQU C.F.=M.multidot.y.multidot.w.sup.2,
where
C.F. is the magnitude of centrifugal force produced by the exciter, PA1 M is the amount of rotating eccentric mass, PA1 y is the distance from the center of gravity of the eccentric mass to the center of rotation, and PA1 w is the angular velocity of the eccentric mass.
Heretofore the expedients that have been employed for varying the magnitude of centrifugal force produced by a centrifugal exciter have all possessed marked disadvantages.
In one such exciter the eccentric mass was a liquid chamber, the effective mass of which was changed by increasing or decreasing the quantity of liquid with which the chamber was filled. To obtain a reasonably small exciter with a satisfactorily high force output, the liquid used was mercury, which is poisonous. The arrangement had the further disadvantage of being somewhat complicated and expensive.
Another prior exciter had multiple eccentric masses. One of these was fixed to the rotating exciter shaft, the other was free to rotate relative to that shaft to positions in which it was either in phase or out of phase with the eccentric mass that was fixed to the shaft, depending upon the direction of shaft rotation. This arrangement was relatively inflexible, especially in view of the fact that a centrifugal exciter for a roller compactor should always be rotated in the direction that corresponds to the direction in which the machine is being propelled.
In another prior centrifugal exciter that had multiple eccentric masses, a relatively large eccentric mass was fixed to the exciter shaft and a smaller mass was mounted on that shaft 180.degree. out of phase with the larger one. Although constrained to rotate with the shaft, the smaller mass was carried for radially in and out movement relative to it and was spring biased radially inward to a normal position adjacent to the shaft. As rotational speed of the shaft increased, centrifugal force on the smaller mass moved it outwardly against its bias so that it increasingly cancelled a portion of the vibratory force produced by the larger mass. With this arrangement the exciter could produce only one level of centrifugal force at a given frequency. It could not be selectively operated at any desired force and frequency combination, as is necessary for optimum compacting effectiveness under all conditions.
As is evident from the relatively unsatisfactory solutions that have heretofore been accepted by the art, the problem of devising a centrifugal exciter with a flexibly adjustable centrifugal force magnitude has been of baffling complexity. Such an exciter, to be satisfactory, must be adjustable while the exciter is in operation; it should be bidirectional; it must be simple and inexpensive; and it must be inherently sturdy and reliable notwithstanding the constant and intense vibration to which all of its parts are subjected when it is in operation.